This invention relates to fuel injection control systems, and more particularly to a fuel injection control system wherein a fuel injection flow rate in each cylinder is controlled in such a manner that the optimum air-fuel ratio is obtainable in each cylinder.
In the fuel injection control system of the type described, a basic fuel injection flow rate, which is extracted from an intake air flow rate and an engine rotational speed, is corrected by various data on the driving conditions of the engine such as engine cooling water temperature, intake air temperature, or a residual oxygen concentration so as to keep the air to fuel ratio at the optimum value, and satisfactory driving performance is maintained. In consequence, unless the intake air flow rate is accurately metered, the air-fuel ratio can not be controlled to the optimum value.
There has been commonly used an intake air flow rate sensor (air flow meter) having a measuring plate which is interposed between an air cleaner and a throttle valve and is rotatably driven commensurate to an air flow into the engine to cunvert a rotational angle (opening degree) of the measuring plate into a voltage so as to detect an intake air flow rate. As shown in FIG. 1, in the air flow meter of the type described, an opening degree of the measuring plate of the air flow meter is logarithmically varied for a differential pressure .DELTA.P between the front and rear sides of the air flow meter in a throttle chamber to maintain a measuring accuracy even when the intake air flow rate is low.
When the engine is fully loaded, a pulsating flow of the intake are generated, in which case, the differential pressure .DELTA.P is varied in a sine-wave form between P.sub.1 and P.sub.2, being centered about P.sub.n as shown in FIG. 1. At this time, the central opening degree of the measuring plate becomes an opening degree b larger in value than a mean value between opening degrees a.sub.0 and a.sub.1 corresponding to pressures P.sub.0 and P.sub.1, and a value larger than the actual intake air flow rate is detected by the air flow meter.
As a result, when the engine is fully loaded, a fuel injection signal making the fuel injection flow rate to be excessively large, i.e., the air-fuel ratio to be over-rich is emitted from the fuel injection control system to an injector, thus resulting in a decreased engine output and a lowered efficiency of purifying an exhaust gas.
To avoid occurrence of the above-described phenomenon, there is a method of reducing the fuel increasing rate under the full-load of the engine, a method of imposing an upper limit to the pulse width of fuel injection pulses under the same, or the like. However, during the transitional condition of the engine such as racing, an over-rich air-fuel ratio is required as compared with that when the engine is in full-load. Therefore, if merely the fuel injection flow rate is set to the optimum value at the time of full-load of the engine, there are presented problems of that the engine does not reach a required rotational speed and so forth.